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Coroutines support libraries for Kotlin
/*
* Copyright 2016-2021 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
*/
package kotlinx.coroutines.selects
import kotlinx.atomicfu.*
import kotlinx.coroutines.*
import kotlinx.coroutines.channels.*
import kotlinx.coroutines.internal.*
import kotlinx.coroutines.selects.TrySelectDetailedResult.*
import kotlin.contracts.*
import kotlin.coroutines.*
import kotlin.internal.*
import kotlin.jvm.*
/**
* Waits for the result of multiple suspending functions simultaneously, which are specified using _clauses_
* in the [builder] scope of this select invocation. The caller is suspended until one of the clauses
* is either _selected_ or _fails_.
*
* At most one clause is *atomically* selected and its block is executed. The result of the selected clause
* becomes the result of the select. If any clause _fails_, then the select invocation produces the
* corresponding exception. No clause is selected in this case.
*
* This select function is _biased_ to the first clause. When multiple clauses can be selected at the same time,
* the first one of them gets priority. Use [selectUnbiased] for an unbiased (randomized) selection among
* the clauses.
* There is no `default` clause for select expression. Instead, each selectable suspending function has the
* corresponding non-suspending version that can be used with a regular `when` expression to select one
* of the alternatives or to perform the default (`else`) action if none of them can be immediately selected.
*
* ### List of supported select methods
*
* | **Receiver** | **Suspending function** | **Select clause**
* | ---------------- | --------------------------------------------- | -----------------------------------------------------
* | [Job] | [join][Job.join] | [onJoin][Job.onJoin]
* | [Deferred] | [await][Deferred.await] | [onAwait][Deferred.onAwait]
* | [SendChannel] | [send][SendChannel.send] | [onSend][SendChannel.onSend]
* | [ReceiveChannel] | [receive][ReceiveChannel.receive] | [onReceive][ReceiveChannel.onReceive]
* | [ReceiveChannel] | [receiveCatching][ReceiveChannel.receiveCatching] | [onReceiveCatching][ReceiveChannel.onReceiveCatching]
* | none | [delay] | [onTimeout][SelectBuilder.onTimeout]
*
* This suspending function is cancellable. If the [Job] of the current coroutine is cancelled or completed while this
* function is suspended, this function immediately resumes with [CancellationException].
* There is a **prompt cancellation guarantee**. If the job was cancelled while this function was
* suspended, it will not resume successfully. See [suspendCancellableCoroutine] documentation for low-level details.
*
* Note that this function does not check for cancellation when it is not suspended.
* Use [yield] or [CoroutineScope.isActive] to periodically check for cancellation in tight loops if needed.
*/
@OptIn(ExperimentalContracts::class)
public suspend inline fun select(crossinline builder: SelectBuilder.() -> Unit): R {
contract {
callsInPlace(builder, InvocationKind.EXACTLY_ONCE)
}
return SelectImplementation(coroutineContext).run {
builder(this)
// TAIL-CALL OPTIMIZATION: the only
// suspend call is at the last position.
doSelect()
}
}
/**
* Scope for [select] invocation.
*
* An instance of [SelectBuilder] can only be retrieved as a receiver of a [select] block call,
* and it is only valid during the registration phase of the select builder.
* Any uses outside it lead to unspecified behaviour and are prohibited.
*
* The general rule of thumb is that instances of this type should always be used
* implicitly and there shouldn't be any signatures mentioning this type,
* whether explicitly (e.g. function signature) or implicitly (e.g. inferred `val` type).
*/
public sealed interface SelectBuilder {
/**
* Registers a clause in this [select] expression without additional parameters that does not select any value.
*/
public operator fun SelectClause0.invoke(block: suspend () -> R)
/**
* Registers clause in this [select] expression without additional parameters that selects value of type [Q].
*/
public operator fun SelectClause1.invoke(block: suspend (Q) -> R)
/**
* Registers clause in this [select] expression with additional parameter of type [P] that selects value of type [Q].
*/
public operator fun SelectClause2.invoke(param: P, block: suspend (Q) -> R)
/**
* Registers clause in this [select] expression with additional nullable parameter of type [P]
* with the `null` value for this parameter that selects value of type [Q].
*/
public operator fun SelectClause2.invoke(block: suspend (Q) -> R): Unit = invoke(null, block)
/**
* Clause that selects the given [block] after a specified timeout passes.
* If timeout is negative or zero, [block] is selected immediately.
*
* **Note: This is an experimental api.** It may be replaced with light-weight timer/timeout channels in the future.
*
* @param timeMillis timeout time in milliseconds.
*/
@ExperimentalCoroutinesApi
@Suppress("INVISIBLE_REFERENCE", "INVISIBLE_MEMBER")
@LowPriorityInOverloadResolution
@Deprecated(
message = "Replaced with the same extension function",
level = DeprecationLevel.ERROR, replaceWith = ReplaceWith(expression = "onTimeout", imports = ["kotlinx.coroutines.selects.onTimeout"])
) // Since 1.7.0, was experimental
public fun onTimeout(timeMillis: Long, block: suspend () -> R): Unit = onTimeout(timeMillis, block)
}
/**
* Each [select] clause is specified with:
* 1) the [object of this clause][clauseObject],
* such as the channel instance for [SendChannel.onSend];
* 2) the function that specifies how this clause
* should be registered in the object above;
* 3) the function that modifies the internal result
* (passed via [SelectInstance.trySelect] or
* [SelectInstance.selectInRegistrationPhase])
* to the argument of the user-specified block.
* 4) the function that specifies how the internal result provided via
* [SelectInstance.trySelect] or [SelectInstance.selectInRegistrationPhase]
* should be processed in case of this `select` cancellation while dispatching.
*/
@InternalCoroutinesApi
public sealed interface SelectClause {
public val clauseObject: Any
public val regFunc: RegistrationFunction
public val processResFunc: ProcessResultFunction
public val onCancellationConstructor: OnCancellationConstructor?
}
/**
* The registration function specifies how the `select` instance should be registered into
* the specified clause object. In case of channels, the registration logic
* coincides with the plain `send/receive` operation with the only difference that
* the `select` instance is stored as a waiter instead of continuation.
*/
@InternalCoroutinesApi
public typealias RegistrationFunction = (clauseObject: Any, select: SelectInstance<*>, param: Any?) -> Unit
/**
* This function specifies how the _internal_ result, provided via [SelectInstance.selectInRegistrationPhase]
* or [SelectInstance.trySelect] should be processed. For example, both [ReceiveChannel.onReceive] and
* [ReceiveChannel.onReceiveCatching] clauses perform exactly the same synchronization logic,
* but differ when the channel has been discovered in the closed or cancelled state.
*/
@InternalCoroutinesApi
public typealias ProcessResultFunction = (clauseObject: Any, param: Any?, clauseResult: Any?) -> Any?
/**
* This function specifies how the internal result, provided via [SelectInstance.trySelect]
* or [SelectInstance.selectInRegistrationPhase], should be processed in case of this `select`
* cancellation while dispatching. Unfortunately, we cannot pass this function only in [SelectInstance.trySelect],
* as [SelectInstance.selectInRegistrationPhase] can be called when the coroutine is already cancelled.
*/
@InternalCoroutinesApi
public typealias OnCancellationConstructor = (select: SelectInstance<*>, param: Any?, internalResult: Any?) -> (Throwable) -> Unit
/**
* Clause for [select] expression without additional parameters that does not select any value.
*/
public sealed interface SelectClause0 : SelectClause
internal class SelectClause0Impl(
override val clauseObject: Any,
override val regFunc: RegistrationFunction,
override val onCancellationConstructor: OnCancellationConstructor? = null
) : SelectClause0 {
override val processResFunc: ProcessResultFunction = DUMMY_PROCESS_RESULT_FUNCTION
}
private val DUMMY_PROCESS_RESULT_FUNCTION: ProcessResultFunction = { _, _, _ -> null }
/**
* Clause for [select] expression without additional parameters that selects value of type [Q].
*/
public sealed interface SelectClause1 : SelectClause
internal class SelectClause1Impl(
override val clauseObject: Any,
override val regFunc: RegistrationFunction,
override val processResFunc: ProcessResultFunction,
override val onCancellationConstructor: OnCancellationConstructor? = null
) : SelectClause1
/**
* Clause for [select] expression with additional parameter of type [P] that selects value of type [Q].
*/
public sealed interface SelectClause2 : SelectClause
internal class SelectClause2Impl(
override val clauseObject: Any,
override val regFunc: RegistrationFunction,
override val processResFunc: ProcessResultFunction,
override val onCancellationConstructor: OnCancellationConstructor? = null
) : SelectClause2
/**
* Internal representation of `select` instance.
*
* @suppress **This is unstable API, and it is subject to change.**
*/
@InternalCoroutinesApi
public sealed interface SelectInstance {
/**
* The context of the coroutine that is performing this `select` operation.
*/
public val context: CoroutineContext
/**
* This function should be called by other operations,
* which are trying to perform a rendezvous with this `select`.
* Returns `true` if the rendezvous succeeds, `false` otherwise.
*
* Note that according to the current implementation, a rendezvous attempt can fail
* when either another clause is already selected or this `select` is still in
* REGISTRATION phase. To distinguish the reasons, [SelectImplementation.trySelectDetailed]
* function can be used instead.
*/
public fun trySelect(clauseObject: Any, result: Any?): Boolean
/**
* When this `select` instance is stored as a waiter, the specified [handle][disposableHandle]
* defines how the stored `select` should be removed in case of cancellation or another clause selection.
*/
public fun disposeOnCompletion(disposableHandle: DisposableHandle)
/**
* When a clause becomes selected during registration, the corresponding internal result
* (which is further passed to the clause's [ProcessResultFunction]) should be provided
* via this function. After that, other clause registrations are ignored and [trySelect] fails.
*/
public fun selectInRegistrationPhase(internalResult: Any?)
}
internal interface SelectInstanceInternal: SelectInstance, Waiter
@PublishedApi
internal open class SelectImplementation(
override val context: CoroutineContext
) : CancelHandler(), SelectBuilder, SelectInstanceInternal {
/**
* Essentially, the `select` operation is split into three phases: REGISTRATION, WAITING, and COMPLETION.
*
* == Phase 1: REGISTRATION ==
* In the first REGISTRATION phase, the user-specified [SelectBuilder] is applied, and all the listed clauses
* are registered via the provided [registration functions][SelectClause.regFunc]. Intuitively, `select` clause
* registration is similar to the plain blocking operation, with the only difference that this [SelectInstance]
* is stored as a waiter instead of continuation, and [SelectInstance.trySelect] is used to make a rendezvous.
* Also, when registering, it is possible for the operation to complete immediately, without waiting. In this case,
* [SelectInstance.selectInRegistrationPhase] should be used. Otherwise, when no rendezvous happens and this `select`
* instance is stored as a waiter, a completion handler for the registering clause should be specified via
* [SelectInstance.disposeOnCompletion]; this handler specifies how to remove this `select` instance from the
* clause object when another clause becomes selected or the operation cancels.
*
* After a clause registration is completed, another coroutine can attempt to make a rendezvous with this `select`.
* However, to resolve a race between clauses registration and [SelectInstance.trySelect], the latter fails when
* this `select` is still in REGISTRATION phase. Thus, the corresponding clause has to be registered again.
*
* In this phase, the `state` field stores either a special [STATE_REG] marker or
* a list of clauses to be re-registered due to failed rendezvous attempts.
*
* == Phase 2: WAITING ==
* If no rendezvous happens in REGISTRATION phase, the `select` operation moves to WAITING one and suspends until
* [SelectInstance.trySelect] is called. Also, when waiting, this `select` can be cancelled. In the latter case,
* further [SelectInstance.trySelect] attempts fail, and all the completion handlers, specified via
* [SelectInstance.disposeOnCompletion], are invoked to remove this `select` instance from the corresponding
* clause objects.
*
* In this phase, the `state` field stores either the continuation to be later resumed or a special `Cancelled`
* object (with the cancellation cause inside) when this `select` becomes cancelled.
*
* == Phase 3: COMPLETION ==
* Once a rendezvous happens either in REGISTRATION phase (via [SelectInstance.selectInRegistrationPhase]) or
* in WAITING phase (via [SelectInstance.trySelect]), this `select` moves to the final `COMPLETION` phase.
* First, the provided internal result is processed via the [ProcessResultFunction] of the selected clause;
* it returns the argument for the user-specified block or throws an exception (see [SendChannel.onSend] as
* an example). After that, this `select` should be removed from all other clause objects by calling the
* corresponding [DisposableHandle]-s, provided via [SelectInstance.disposeOnCompletion] during registration.
* At the end, the user-specified block is called and this `select` finishes.
*
* In this phase, once a rendezvous is happened, the `state` field stores the corresponding clause.
* After that, it moves to [STATE_COMPLETED] to avoid memory leaks.
*
*
*
* The state machine is listed below:
*
* REGISTRATION PHASE WAITING PHASE COMPLETION PHASE
* ⌢⌢⌢⌢⌢⌢⌢⌢⌢⌢⌢⌢⌢⌢⌢⌢⌢⌢⌢⌢ ⌢⌢⌢⌢⌢⌢⌢⌢⌢⌢⌢⌢ ⌢⌢⌢⌢⌢⌢⌢⌢⌢⌢⌢⌢⌢⌢
*
* +-----------+ +-----------+
* | CANCELLED | | COMPLETED |
* +-----------+ +-----------+
* ^ ^
* INITIAL STATE | | this `select`
* ------------+ | cancelled | is completed
* \ | |
* +=============+ move to +------+ successful +------------+
* +--| STATE_REG |---------------> | cont |-----------------| ClauseData |
* | +=============+ WAITING phase +------+ trySelect(..) +------------+
* | ^ | ^
* | | | some clause has been selected during registration |
* add a | | +-------------------------------------------------------+
* clause to be | | |
* re-registered | | re-register some clause has been selected |
* | | clauses during registration while there |
* v | are clauses to be re-registered; |
* +------------------+ ignore the latter |
* +--| List |-----------------------------------------------------+
* | +------------------+
* | ^
* | | add one more clause
* | | for re-registration
* +------------+
*
* One of the most valuable benefits of this `select` design is that it allows processing clauses
* in a way similar to plain operations, such as `send` or `receive` on channels. The only difference
* is that instead of continuation, the operation should store the provided `select` instance object.
* Thus, this design makes it possible to support the `select` expression for any blocking data structure
* in Kotlin Coroutines.
*
* It is worth mentioning that the algorithm above provides "obstruction-freedom" non-blocking guarantee
* instead of the standard "lock-freedom" to avoid using heavy descriptors. In practice, this relaxation
* does not make significant difference. However, it is vital for Kotlin Coroutines to provide some
* non-blocking guarantee, as users may add blocking code in [SelectBuilder], and this blocking code
* should not cause blocking behaviour in other places, such as an attempt to make a rendezvous with
* the `select` that is hang in REGISTRATION phase.
*
* Also, this implementation is NOT linearizable under some circumstances. The reason is that a rendezvous
* attempt with `select` (via [SelectInstance.trySelect]) may fail when this `select` operation is still
* in REGISTRATION phase. Consider the following situation on two empty rendezvous channels `c1` and `c2`
* and the `select` operation that tries to send an element to one of these channels. First, this `select`
* instance is registered as a waiter in `c1`. After that, another thread can observe that `c1` is no longer
* empty and try to receive an element from `c1` -- this receive attempt fails due to the `select` operation
* being in REGISTRATION phase.
* It is also possible to observe that this `select` operation registered in `c2` first, and only after that in
* `c1` (it has to re-register in `c1` after the unsuccessful rendezvous attempt), which is also non-linearizable.
* We, however, find such a non-linearizable behaviour not so important in practice and leverage the correctness
* relaxation for the algorithm simplicity and the non-blocking progress guarantee.
*/
/**
* The state of this `select` operation. See the description above for details.
*/
private val state = atomic(STATE_REG)
/**
* Returns `true` if this `select` instance is in the REGISTRATION phase;
* otherwise, returns `false`.
*/
private val inRegistrationPhase
get() = state.value.let {
it === STATE_REG || it is List<*>
}
/**
* Returns `true` if this `select` is already selected;
* thus, other parties are bound to fail when making a rendezvous with it.
*/
private val isSelected
get() = state.value is SelectImplementation<*>.ClauseData
/**
* Returns `true` if this `select` is cancelled.
*/
private val isCancelled
get() = state.value === STATE_CANCELLED
/**
* List of clauses waiting on this `select` instance.
*
* This property is the subject to bening data race: concurrent cancellation might null-out this property
* while [trySelect] operation reads it and iterates over its content.
* A logical race is resolved by the consensus on [state] property.
*/
@BenignDataRace
private var clauses: MutableList? = ArrayList(2)
/**
* Stores the completion action provided through [disposeOnCompletion] or [invokeOnCancellation]
* during clause registration. After that, if the clause is successfully registered
* (so, it has not completed immediately), this handler is stored into
* the corresponding [ClauseData] instance.
*
* Note that either [DisposableHandle] is provided, or a [Segment] instance with
* the index in it, which specify the location of storing this `select`.
* In the latter case, [Segment.onCancellation] should be called on completion/cancellation.
*/
private var disposableHandleOrSegment: Any? = null
/**
* In case the disposable handle is specified via [Segment]
* and index in it, implying calling [Segment.onCancellation],
* the corresponding index is stored in this field.
* The segment is stored in [disposableHandleOrSegment].
*/
private var indexInSegment: Int = -1
/**
* Stores the result passed via [selectInRegistrationPhase] during clause registration
* or [trySelect], which is called by another coroutine trying to make a rendezvous
* with this `select` instance. Further, this result is processed via the
* [ProcessResultFunction] of the selected clause.
*
* Unfortunately, we cannot store the result in the [state] field, as the latter stores
* the clause object upon selection (see [ClauseData.clauseObject] and [SelectClause.clauseObject]).
* Instead, it is possible to merge the [internalResult] and [disposableHandle] fields into
* one that stores either result when the clause is successfully registered ([inRegistrationPhase] is `true`),
* or [DisposableHandle] instance when the clause is completed during registration ([inRegistrationPhase] is `false`).
* Yet, this optimization is omitted for code simplicity.
*
* This property is the subject to benign data race:
* [Cleanup][cleanup] procedure can be invoked both as part of the completion sequence
* and as a cancellation handler triggered by an external cancellation.
* In both scenarios, [NO_RESULT] is written to this property via race.
*/
@BenignDataRace
private var internalResult: Any? = NO_RESULT
/**
* This function is called after the [SelectBuilder] is applied. In case one of the clauses is already selected,
* the algorithm applies the corresponding [ProcessResultFunction] and invokes the user-specified [block][ClauseData.block].
* Otherwise, it moves this `select` to WAITING phase (re-registering clauses if needed), suspends until a rendezvous
* is happened, and then completes the operation by applying the corresponding [ProcessResultFunction] and
* invoking the user-specified [block][ClauseData.block].
*/
@PublishedApi
internal open suspend fun doSelect(): R =
if (isSelected) complete() // Fast path
else doSelectSuspend() // Slow path
// We separate the following logic as it has two suspension points
// and, therefore, breaks the tail-call optimization if it were
// inlined in [doSelect]
private suspend fun doSelectSuspend(): R {
// In case no clause has been selected during registration,
// the `select` operation suspends and waits for a rendezvous.
waitUntilSelected() // <-- suspend call => no tail-call optimization here
// There is a selected clause! Apply the corresponding
// [ProcessResultFunction] and invoke the user-specified block.
return complete() // <-- one more suspend call
}
// ========================
// = CLAUSES REGISTRATION =
// ========================
override fun SelectClause0.invoke(block: suspend () -> R) =
ClauseData(clauseObject, regFunc, processResFunc, PARAM_CLAUSE_0, block, onCancellationConstructor).register()
override fun SelectClause1.invoke(block: suspend (Q) -> R) =
ClauseData(clauseObject, regFunc, processResFunc, null, block, onCancellationConstructor).register()
override fun SelectClause2.invoke(param: P, block: suspend (Q) -> R) =
ClauseData(clauseObject, regFunc, processResFunc, param, block, onCancellationConstructor).register()
/**
* Attempts to register this `select` clause. If another clause is already selected,
* this function does nothing and completes immediately.
* Otherwise, it registers this `select` instance in
* the [clause object][ClauseData.clauseObject]
* according to the provided [registration function][ClauseData.regFunc].
* On success, this `select` instance is stored as a waiter
* in the clause object -- the algorithm also stores
* the provided via [disposeOnCompletion] completion action
* and adds the clause to the list of registered one.
* In case of registration failure, the internal result
* (not processed by [ProcessResultFunction] yet) must be
* provided via [selectInRegistrationPhase] -- the algorithm
* updates the state to this clause reference.
*/
@JvmName("register")
internal fun ClauseData.register(reregister: Boolean = false) {
assert { state.value !== STATE_CANCELLED }
// Is there already selected clause?
if (state.value.let { it is SelectImplementation<*>.ClauseData }) return
// For new clauses, check that there does not exist
// another clause with the same object.
if (!reregister) checkClauseObject(clauseObject)
// Try to register in the corresponding object.
if (tryRegisterAsWaiter(this@SelectImplementation)) {
// Successfully registered, and this `select` instance
// is stored as a waiter. Add this clause to the list
// of registered clauses and store the provided via
// [invokeOnCompletion] completion action into the clause.
//
// Importantly, the [waitUntilSelected] function is implemented
// carefully to ensure that the cancellation handler has not been
// installed when clauses re-register, so the logic below cannot
// be invoked concurrently with the clean-up procedure.
// This also guarantees that the list of clauses cannot be cleared
// in the registration phase, so it is safe to read it with "!!".
if (!reregister) clauses!! += this
disposableHandleOrSegment = [email protected]
indexInSegment = [email protected]
[email protected] = null
[email protected] = -1
} else {
// This clause has been selected!
// Update the state correspondingly.
state.value = this
}
}
/**
* Checks that there does not exist another clause with the same object.
*/
private fun checkClauseObject(clauseObject: Any) {
// Read the list of clauses, it is guaranteed that it is non-null.
// In fact, it can become `null` only in the clean-up phase, while
// this check can be called only in the registration one.
val clauses = clauses!!
// Check that there does not exist another clause with the same object.
check(clauses.none { it.clauseObject === clauseObject }) {
"Cannot use select clauses on the same object: $clauseObject"
}
}
override fun disposeOnCompletion(disposableHandle: DisposableHandle) {
this.disposableHandleOrSegment = disposableHandle
}
/**
* An optimized version for the code below that does not allocate
* a cancellation handler object and efficiently stores the specified
* [segment] and [index].
*
* ```
* disposeOnCompletion {
* segment.onCancellation(index, null)
* }
* ```
*/
override fun invokeOnCancellation(segment: Segment<*>, index: Int) {
this.disposableHandleOrSegment = segment
this.indexInSegment = index
}
override fun selectInRegistrationPhase(internalResult: Any?) {
this.internalResult = internalResult
}
// =========================
// = WAITING FOR SELECTION =
// =========================
/**
* Suspends and waits until some clause is selected. However, it is possible for a concurrent
* coroutine to invoke [trySelect] while this `select` is still in REGISTRATION phase.
* In this case, [trySelect] marks the corresponding select clause to be re-registered, and
* this function performs registration of such clauses. After that, it atomically stores
* the continuation into the [state] field if there is no more clause to be re-registered.
*/
private suspend fun waitUntilSelected() = suspendCancellableCoroutine sc@ { cont ->
// Update the state.
state.loop { curState ->
when {
// This `select` is in REGISTRATION phase, and there is no clause to be re-registered.
// Perform a transition to WAITING phase by storing the current continuation.
curState === STATE_REG -> if (state.compareAndSet(curState, cont)) {
// Perform a clean-up in case of cancellation.
//
// Importantly, we MUST install the cancellation handler
// only when the algorithm is bound to suspend. Otherwise,
// a race with [tryRegister] is possible, and the provided
// via [disposeOnCompletion] cancellation action can be ignored.
// Also, we MUST guarantee that this dispose handle is _visible_
// according to the memory model, and we CAN guarantee this when
// the state is updated.
cont.invokeOnCancellation(this.asHandler)
return@sc
}
// This `select` is in REGISTRATION phase, but there are clauses that has to be registered again.
// Perform the required registrations and try again.
curState is List<*> -> if (state.compareAndSet(curState, STATE_REG)) {
@Suppress("UNCHECKED_CAST")
curState as List
curState.forEach { reregisterClause(it) }
}
// This `select` operation became completed during clauses re-registration.
curState is SelectImplementation<*>.ClauseData -> {
cont.resume(Unit, curState.createOnCancellationAction(this, internalResult))
return@sc
}
// This `select` cannot be in any other state.
else -> error("unexpected state: $curState")
}
}
}
/**
* Re-registers the clause with the specified
* [clause object][clauseObject] after unsuccessful
* [trySelect] of this clause while the `select`
* was still in REGISTRATION phase.
*/
private fun reregisterClause(clauseObject: Any) {
val clause = findClause(clauseObject)!! // it is guaranteed that the corresponding clause is presented
clause.disposableHandleOrSegment = null
clause.indexInSegment = -1
clause.register(reregister = true)
}
// ==============
// = RENDEZVOUS =
// ==============
override fun trySelect(clauseObject: Any, result: Any?): Boolean =
trySelectInternal(clauseObject, result) == TRY_SELECT_SUCCESSFUL
/**
* Similar to [trySelect] but provides a failure reason
* if this rendezvous is unsuccessful. We need this function
* in the channel implementation.
*/
fun trySelectDetailed(clauseObject: Any, result: Any?) =
TrySelectDetailedResult(trySelectInternal(clauseObject, result))
private fun trySelectInternal(clauseObject: Any, internalResult: Any?): Int {
while (true) {
when (val curState = state.value) {
// Perform a rendezvous with this select if it is in WAITING state.
is CancellableContinuation<*> -> {
val clause = findClause(clauseObject) ?: continue // retry if `clauses` is already `null`
val onCancellation = clause.createOnCancellationAction(this@SelectImplementation, internalResult)
if (state.compareAndSet(curState, clause)) {
@Suppress("UNCHECKED_CAST")
val cont = curState as CancellableContinuation
// Success! Store the resumption value and
// try to resume the continuation.
this.internalResult = internalResult
if (cont.tryResume(onCancellation)) return TRY_SELECT_SUCCESSFUL
// If the resumption failed, we need to clean the [result] field to avoid memory leaks.
this.internalResult = NO_RESULT
return TRY_SELECT_CANCELLED
}
}
// Already selected.
STATE_COMPLETED, is SelectImplementation<*>.ClauseData -> return TRY_SELECT_ALREADY_SELECTED
// Already cancelled.
STATE_CANCELLED -> return TRY_SELECT_CANCELLED
// This select is still in REGISTRATION phase, re-register the clause
// in order not to wait until this select moves to WAITING phase.
// This is a rare race, so we do not need to worry about performance here.
STATE_REG -> if (state.compareAndSet(curState, listOf(clauseObject))) return TRY_SELECT_REREGISTER
// This select is still in REGISTRATION phase, and the state stores a list of clauses
// for re-registration, add the selecting clause to this list.
// This is a rare race, so we do not need to worry about performance here.
is List<*> -> if (state.compareAndSet(curState, curState + clauseObject)) return TRY_SELECT_REREGISTER
// Another state? Something went really wrong.
else -> error("Unexpected state: $curState")
}
}
}
/**
* Finds the clause with the corresponding [clause object][SelectClause.clauseObject].
* If the reference to the list of clauses is already cleared due to completion/cancellation,
* this function returns `null`
*/
private fun findClause(clauseObject: Any): ClauseData? {
// Read the list of clauses. If the `clauses` field is already `null`,
// the clean-up phase has already completed, and this function returns `null`.
val clauses = this.clauses ?: return null
// Find the clause with the specified clause object.
return clauses.find { it.clauseObject === clauseObject }
?: error("Clause with object $clauseObject is not found")
}
// ==============
// = COMPLETION =
// ==============
/**
* Completes this `select` operation after the internal result is provided
* via [SelectInstance.trySelect] or [SelectInstance.selectInRegistrationPhase].
* (1) First, this function applies the [ProcessResultFunction] of the selected clause
* to the internal result.
* (2) After that, the [clean-up procedure][cleanup]
* is called to remove this `select` instance from other clause objects, and
* make it possible to collect it by GC after this `select` finishes.
* (3) Finally, the user-specified block is invoked
* with the processed result as an argument.
*/
private suspend fun complete(): R {
assert { isSelected }
// Get the selected clause.
@Suppress("UNCHECKED_CAST")
val selectedClause = state.value as SelectImplementation.ClauseData
// Perform the clean-up before the internal result processing and
// the user-specified block invocation to guarantee the absence
// of memory leaks. Collect the internal result before that.
val internalResult = this.internalResult
cleanup(selectedClause)
// Process the internal result and invoke the user's block.
return if (!RECOVER_STACK_TRACES) {
// TAIL-CALL OPTIMIZATION: the `suspend` block
// is invoked at the very end.
val blockArgument = selectedClause.processResult(internalResult)
selectedClause.invokeBlock(blockArgument)
} else {
// TAIL-CALL OPTIMIZATION: the `suspend`
// function is invoked at the very end.
// However, internally this `suspend` function
// constructs a state machine to recover a
// possible stack-trace.
processResultAndInvokeBlockRecoveringException(selectedClause, internalResult)
}
}
private suspend fun processResultAndInvokeBlockRecoveringException(clause: ClauseData, internalResult: Any?): R =
try {
val blockArgument = clause.processResult(internalResult)
clause.invokeBlock(blockArgument)
} catch (e: Throwable) {
// In the debug mode, we need to properly recover
// the stack-trace of the exception; the tail-call
// optimization cannot be applied here.
recoverAndThrow(e)
}
/**
* Invokes all [DisposableHandle]-s provided via
* [SelectInstance.disposeOnCompletion] during
* clause registrations.
*/
private fun cleanup(selectedClause: ClauseData) {
assert { state.value == selectedClause }
// Read the list of clauses. If the `clauses` field is already `null`,
// a concurrent clean-up procedure has already completed, and it is safe to finish.
val clauses = this.clauses ?: return
// Invoke all cancellation handlers except for the
// one related to the selected clause, if specified.
clauses.forEach { clause ->
if (clause !== selectedClause) clause.dispose()
}
// We do need to clean all the data to avoid memory leaks.
this.state.value = STATE_COMPLETED
this.internalResult = NO_RESULT
this.clauses = null
}
// [CompletionHandler] implementation, must be invoked on cancellation.
override fun invoke(cause: Throwable?) {
// Update the state.
state.update { cur ->
// Finish immediately when this `select` is already completed.
// Notably, this select might be logically completed
// (the `state` field stores the selected `ClauseData`),
// while the continuation is already cancelled.
// We need to invoke the cancellation handler in this case.
if (cur === STATE_COMPLETED) return
STATE_CANCELLED
}
// Read the list of clauses. If the `clauses` field is already `null`,
// a concurrent clean-up procedure has already completed, and it is safe to finish.
val clauses = this.clauses ?: return
// Remove this `select` instance from all the clause object (channels, mutexes, etc.).
clauses.forEach { it.dispose() }
// We do need to clean all the data to avoid memory leaks.
this.internalResult = NO_RESULT
this.clauses = null
}
/**
* Each `select` clause is internally represented with a [ClauseData] instance.
*/
internal inner class ClauseData(
@JvmField val clauseObject: Any, // the object of this `select` clause: Channel, Mutex, Job, ...
private val regFunc: RegistrationFunction,
private val processResFunc: ProcessResultFunction,
private val param: Any?, // the user-specified param
private val block: Any, // the user-specified block, which should be called if this clause becomes selected
@JvmField val onCancellationConstructor: OnCancellationConstructor?
) {
@JvmField var disposableHandleOrSegment: Any? = null
@JvmField var indexInSegment: Int = -1
/**
* Tries to register the specified [select] instance in [clauseObject] and check
* whether the registration succeeded or a rendezvous has happened during the registration.
* This function returns `true` if this [select] is successfully registered and
* is _waiting_ for a rendezvous, or `false` when this clause becomes
* selected during registration.
*
* For example, the [Channel.onReceive] clause registration
* on a non-empty channel retrieves the first element and completes
* the corresponding [select] via [SelectInstance.selectInRegistrationPhase].
*/
fun tryRegisterAsWaiter(select: SelectImplementation): Boolean {
assert { select.inRegistrationPhase || select.isCancelled }
assert { select.internalResult === NO_RESULT }
regFunc(clauseObject, select, param)
return select.internalResult === NO_RESULT
}
/**
* Processes the internal result provided via either
* [SelectInstance.selectInRegistrationPhase] or
* [SelectInstance.trySelect] and returns an argument
* for the user-specified [block].
*
* Importantly, this function may throw an exception
* (e.g., when the channel is closed in [Channel.onSend], the
* corresponding [ProcessResultFunction] is bound to fail).
*/
fun processResult(result: Any?) = processResFunc(clauseObject, param, result)
/**
* Invokes the user-specified block and returns
* the final result of this `select` clause.
*/
@Suppress("UNCHECKED_CAST")
suspend fun invokeBlock(argument: Any?): R {
val block = block
// We distinguish no-argument and 1-argument
// lambdas via special markers for the clause
// parameters. Specifically, PARAM_CLAUSE_0
// is always used with [SelectClause0], which
// takes a no-argument lambda.
//
// TAIL-CALL OPTIMIZATION: we invoke
// the `suspend` block at the very end.
return if (this.param === PARAM_CLAUSE_0) {
block as suspend () -> R
block()
} else {
block as suspend (Any?) -> R
block(argument)
}
}
fun dispose() {
with(disposableHandleOrSegment) {
if (this is Segment<*>) {
this.onCancellation(indexInSegment, null, context)
} else {
(this as? DisposableHandle)?.dispose()
}
}
}
fun createOnCancellationAction(select: SelectInstance<*>, internalResult: Any?) =
onCancellationConstructor?.invoke(select, param, internalResult)
}
}
private fun CancellableContinuation.tryResume(onCancellation: ((cause: Throwable) -> Unit)?): Boolean {
val token = tryResume(Unit, null, onCancellation) ?: return false
completeResume(token)
return true
}
// trySelectInternal(..) results.
private const val TRY_SELECT_SUCCESSFUL = 0
private const val TRY_SELECT_REREGISTER = 1
private const val TRY_SELECT_CANCELLED = 2
private const val TRY_SELECT_ALREADY_SELECTED = 3
// trySelectDetailed(..) results.
internal enum class TrySelectDetailedResult {
SUCCESSFUL, REREGISTER, CANCELLED, ALREADY_SELECTED
}
private fun TrySelectDetailedResult(trySelectInternalResult: Int): TrySelectDetailedResult = when(trySelectInternalResult) {
TRY_SELECT_SUCCESSFUL -> SUCCESSFUL
TRY_SELECT_REREGISTER -> REREGISTER
TRY_SELECT_CANCELLED -> CANCELLED
TRY_SELECT_ALREADY_SELECTED -> ALREADY_SELECTED
else -> error("Unexpected internal result: $trySelectInternalResult")
}
// Markers for REGISTRATION, COMPLETED, and CANCELLED states.
private val STATE_REG = Symbol("STATE_REG")
private val STATE_COMPLETED = Symbol("STATE_COMPLETED")
private val STATE_CANCELLED = Symbol("STATE_CANCELLED")
// As the selection result is nullable, we use this special
// marker for the absence of result.
private val NO_RESULT = Symbol("NO_RESULT")
// We use this marker parameter objects to distinguish
// SelectClause[0,1,2] and invoke the user-specified block correctly.
internal val PARAM_CLAUSE_0 = Symbol("PARAM_CLAUSE_0")